The present invention relates to a lubricating oil for non-stage transmissions (hereinafter may be referred to as “continuously variable transmissions”), more specifically to a lubricating oil for continuously variable transmissions which has a high traction coefficient even at high temperature and is endowed with a good low temperature fluidity and which is suited as a lubricating oil for continuously variable transmissions for automobiles.
Related Art
A continuously variable transmission (hereinafter referred to as CVT), particularly CVT for automobiles has such severe use conditions that a torque transmission capacity is large and that a fluctuation range of temperature is wide, and therefore a lubricating oil used for a continuously variable transmission is required to have a high traction coefficient over a use temperature range in order to achieve sufficiently high power transmission. Accordingly, since a lubricating oil is reduced usually in a traction coefficient as an oil temperature rises, a lowest value of a traction coefficient of a lubricating oil for a continuously variable transmission, that is, a traction coefficient thereof at high temperature (120° C.) is required to be sufficiently higher than a design value of CVT.
Further, a lubricating oil for a continuously variable transmission assumes a role of a normal lubricating oil in CVT, and therefore it has to have such a high viscosity that a satisfactory oil film can be maintained even at high temperature.
On the other hand, a lubricating oil is required to have a low viscosity (low temperature fluidity) even at low temperature in order to start engines at low temperature in cold districts such as North America, North Europe and the like. That is, a lubricating oil for a continuously variable transmission has to be small in a change of a viscosity caused by a temperature change, in other words, a viscosity index has to be high.
Under such circumstances, the present inventors disclosed previously a fluid for traction drive (refer to a patent document 1) prepared by using a synthetic oil having specific physical properties as a base oil and a lubricant base oil (refer to a patent document 2) which contains at least one hydrocarbon compound having a specific structure as a basic skeleton and which has a high traction coefficient at high temperature, a low viscosity at −40° C. and a high viscosity index.
On the other hand, regulations on fuel consumption are being strengthened in many countries in recent years because of a rise in concerns about environmental problems. CVT tends to be employed in order to meet an increase in needs for a rise in fuel consumption. Since a speed can be varied at a single step in CVT, an optimum engine revolution can be selected based on a required output torque, and a fuel consumption improving effect is large. CVT includes a metal belt system, a chain system, a traction drive system and the like, and a high transmission efficiency is required in all systems. Then, lubricating oils having a high traction coefficient are required to be developed, and the transmission efficiency has to be enhanced.
Also, since a speed is varied at a single step in CVT, it does not have a gear change shock and is free from a fall in an engine revolution in elevating a shift, and therefore it is improved in an acceleration performance and excellent in a drivability.
Further, CVT is loaded as well in large-sized automobiles and trucks in increasing examples. Such large-sized automobiles have a high torque capacity, and therefore lubricating oils having a higher traction coefficient than ever are desired to be developed.    Patent document 1: Japanese Patent Application Laid-Open No. 17280/2000    Patent document 2: WO 2005/035699